A polar maximum of electron concentration at 1000 km altitude

Abstract

Satellite probe measurements from a circular, polar orbit at 1000 km altitude have revealed a broad enhancement of ionization within the polar cap. The latitude of the maximum concentration closely follows the seasonal movement of the solar terminator across the polar cap. The peak is bounded on its nightside by a solar zenith angle of about 110° and on its dayside by a broad midlatitude trough near 60° geomagnetic latitude. Thus one boundary of the peak, and the location of the maximum, is strongly influenced by solar zenith angle while the other boundary is geomagnetically fixed. Although the ionization and heating observed in the peak clearly arises from solar radiation, the most intriguing question is why a peak should be formed at all since this cannot be produced by zenith angle variations alone. The answer appears to lie in the mechanisms which produce the midlatitude trough on the dayside. To explain the trough we invoke the effects of neutral atmospheric winds flowing poleward on the dayside of the Earth in response to solar heating. The observations are shown to agree quantitatively with solutions of the energy and particle equations of the plasma when a neutral wind field derived from a Jacchia model atmosphere is introduced and suitable allowance is made for an upward proton flux at high latitudes. We also conclude that the polar peak discussed here and the one observed by Nishida and interpreted as a neutral point phenomenon may actually be the same feature. Nishida's study employed only data acquired near equinox, thus he could not resolve the seasonal movement we describe here.